Advanced composites of layers of polymer or resin-impregnated fibers are commonly used as a primary structural component in the manufacture of a variety of structures, including airframe components for various types of aircraft. These materials provide greater structural efficiency at lower weights than equivalent metallic structures. Other uses for composite laminates include marine craft, submersibles, land vehicles, stationary structures, and many other applications in mobile or stationary structures or components.
One particular application for which composite laminates are used is the fabrication of airframe components for pilotless aircraft, including pilotless target aircraft or drones, which are often used for the training of military pilots. Given that drones are subject to damage or destruction when used for their intended purpose, it is important to develop manufacturing techniques that can produce drone aircraft as inexpensively as possible. For this reason, composites have become a popular choice for drone airframe fabrication. In support of low cost manufacture, it is also important to reduce the need for manual steps in the assembly of the finished airframe. Manual steps are labor-intensive, expensive, and can lead to finished products that are not uniform.
Drones used for military training are often required to model the physical, visual, thermal, and electromagnetic characteristics of the hostile aircraft against which pilots are being trained. For this reason, drones must be manufactured to specific specifications as mandated by a government agency or civilian organization. One of these specifications defines a particular radar signature that a drone must present in flight. To meet radar signature specifications, drone manufacturers can design a drone to be of a particular size and shape. Additionally, the drone can be constructed with radar absorbing material (RAM) covering all or a portion of the exterior of the airframe. The use of RAM enables a drone manufacturer to precisely configure a drone with a desired level of radar reflectivity. The purpose of this material is to absorb radio frequency radiation (e.g. microwave or radar) to prevent reflection. Some form of RAM is commonly used on military aircraft, ships, land vehicles, and fixed installations.
Many types of radar absorbing materials are known in the art. For example, the U.S. Patents referenced below describe a few RAM compounds that can be used for configuring an object with a desired radar signature. In other implementations, RAM is made from a non-electrically conductive (dielectric) polymer with dispersed particles of conductive and/or magnetic particles (typically a form of Iron). Traditionally, this material is made from a cured elastomeric (rubber-like) material with an adhesive backing to allow installation on the structure that requires this treatment.
Unfortunately, these conventional RAM compounds/materials and associated manufacturing techniques are problematic for several reasons. Using one conventional technique, RAM is sprayed or painted on the exterior of an airframe. However, this technique can only produce a thin layer of RAM on the airframe. If applied too thickly, the RAM is subject to cracking or flaking due to vibration and high airflow in flight. Unfortunately, a thin layer of RAM often cannot produce a desired level of radar absorption.
Using another conventional technique, RAM is manually applied to the exterior of an airframe in strips or pieces cut from a sheet of RAM. The RAM pieces are typically glued, stapled, or riveted to the airframe. Although this technique can achieve a desired thickness and arrangement of RAM on an airframe, the seams or joins between RAM pieces can peel up, form gaps, or perturb the smooth flow of air across the airframe. Further, this technique does not produce a finished product on which the RAM is tightly and uniformly contoured to the mold shape. The finished structure with a RAM layer applied using conventional techniques is not sufficiently durable as the glued-on RAM can create a path for water, air, or other matter that may cause separation of the RAM from the outer surface of the structure. In addition, this technique for applying RAM is labor-intensive and time-consuming.
Using still another conventional technique, RAM is integrated into a formulation of composite material from which a composite laminate airframe is fabricated. This technique avoids the problems associated with sprayed-on or glued-on RAM. However, it is sometimes difficult to achieve a desired level of radar absorption with composite-integrated RAM. Further, it is not possible to cover only a portion of the airframe using this technique. Finally, the integrated RAM can interfere with the structural integrity of the composite airframe.
U.S. Pat. No. 6,486,822 describes coated ferromagnetic particles, which are useful as radar absorbing material (RAM). In particular, ferromagnetic particles such as iron, carbonyl iron, cobalt, nickel, and alloys thereof are provided that have been coated with a protective non-conducting material such as silicon, silicon dioxide, aluminum oxide, and the like. The ferromagnetic particles are coated in a rotating retort containing a gaseous composition that deposits onto or diffuses into the particle. The coated particles are particularly suitable for incorporation into RAM coating compositions intended for use in corrosive atmospheres.
U.S. Pat. No. 5,552,455 describes a radar absorbing material and a process for making same. In detail, the technique includes a binder material containing a mixture of two groups of spheres made of a magnetic material, The first group of spheres have an average diameter and the second group have an average diameter generally 0.73 times the average diameter of the spheres of the first group. The first and second group contains generally equal numbers of spheres. The amount of the binder material incorporated is sufficient to both bind mixture together while maintaining the individual spheres separated from each other.
U.S. Pat. No. 6,411,248 describes a glue-gun applied hot-melt radar-absorbing material (RAM) and method. The hot-melt radar-absorbing material composition comprises: (a) 70 to 85 wt % carbonyl iron powder; (b) 2 to 10 wt % of a metal deactivator; and (c) balance a thermoplastic polyurethane. The method for repair of a body with a radar-absorbing material, comprises: (a) formulating the hot-melt radar-absorbing material of the present invention; (b) forming the hot-melt radar-absorbing material into a shape; (c) applying the hot-melt radar-absorbing material in a molten state onto the body; and (d) allowing the hot-melt radar-absorbing material to cool to room temperature. The shape of the hot-melt RAM is advantageously a “glue stick”, which is configured to go into a glue gun. The repair operator loads the glue stick into the glue gun and pulls the trigger. The glue gun heats the glue stick, and the molten material is applied to the area to be repaired.
U.S. Pat. No. 6,111,534 describes a structural composite material able to absorb radar waves at frequencies of 18 GHz, 35 GHz and 94 GHz. This material comprises at least three layers of non-magnetic, dielectric material obtained by stacks of impregnated plies, including an outer layer with a low reflection index and losses having an effective dielectric permittivity of around 3, to promote the penetration of the incident radar waves, an intermediate layer having an effective dielectric permittivity of around 5, and an inner layer loaded with electrically conductive particles and having a substantial effective dielectric permittivity of around 15 to 20. The material may have applications in the manufacture of chests for military vehicles, for example.
U.S. Pat. No. 7,112,299 describes a method of fabricating laminate articles. A plurality of support templates are arranged to define a part outline corresponding to the laminate article. An outer surface of a primary panel to is secured to the plurality of templates. A secondary panel is arranged in a desired relationship with the primary panel. A vacuum bag is secured to the primary panel to define a vacuum chamber. A vacuum is applied to the vacuum chamber to remove air from between the at least one primary panel and the at least one secondary panel. Optionally, at least one locater peg may be secured to the primary panel and at least one locater hole may be formed in the secondary panel. In this case, the secondary panel is displaced relative to the primary panel such that the at least one locater peg enters the at least one locater hole.
Thus, systems and methods for fabricating composite laminate structures with co-laminated radar absorbing material are needed.